My attempts to make a rapid prototyping machine that I will use to make parts for a machine that will be able to make parts for a copy of itself.

Monday, 24 May 2010

Black and White

I bought some new ABS filament from reprapsource.com as it is a reasonable price, the postage from Germany is not too bad and being in the EU there are no customs charges, so it does not get held to ransom by Parcel Force for their ridiculous handling charge.

The advert does not state a colour so I assumed it would be natural, however when it came it wasn't like any ABS I had encountered before. Natural ABS is cream coloured and opaque. This was white and a bit translucent. At first I though it was HDPE, but when bent it bruised, which is a characteristic of ABS.

I ran it first in HydraRaptor. The only issue I had was that it didn't want to stick to the PET tape I was using until I raised the bed temperature to 140°C for the first layer and extruded at 250°C. For subsequent layers I revert to the bed at 110°C and filament at 240°C.

The objects produced look nice in white and seem to be harder than those made in natural. I don't think it is simply pigmented ABS, I think it is a different formulation.

My impressions of using PET tape instead of Kapton tape is that it doesn't seem to give as much grip as new Kapton, but it doesn't degrade. I can make most things on it with HydraRaptor without any warping at all, but Mendel bed springs tend to come unstuck. This is because they are relatively tall and have very little contact area with the bed. If the extruder hits a slight blob on a high layer it will snap the part off. Sometimes the loose part hits another part and starts a chain reaction where they all fall off.

When doing raft-less builds on PET or Kapton it is essential that the first layer outline sticks perfectly and has no gaps in it, especially at the corners. If the first layer is too high it obviously doesn't stick and takes short cuts across the corners. If it is too low it also lifts at the corners though. What happens is that the filament becomes squashed into a flat ribbon. When that tries to bend around a sharp corner the outside has to stretch but instead it lifts and folds over inwards. A difference in z-value of 0.05mm can make all the difference. Increasing the temperature also helps to make the plastic bend around corners. If a corner does not stick perfectly then after two or three layers it will curl up at an angle of about 45°. This effect is not like the corner warping you get on a cold bed. It is much more localised and extreme. Small objects tend to come off during the build if a corner lifts.

With the natural ABS I was using before on Kapton it was far less critical. Objects stuck so well I had to remove them with a hammer or use a flexible bed. With white ABS on PET tape the objects can be removed more easily. Sometimes they just come free when they are cooled.

When I tried the new ABS in my Mendel it took a lot more tweaking to get it to work. The first issue was that I had to increase the feed rate by about 18% relative to what I was using for PLA. My theory is that being softer it presses further into the threaded pulley and so sees a smaller pulley diameter. The hobbed M8 bolt has an internal radius of only about 5mm. The drive pulley on HydraRaptor is about twice that diameter and seems give more grip on softer plastics and doesn't need the 18% bodge factor when switching from PLA to ABS. I just tell it the filament diameter and it just works.

The next problem I had was that holes tended to shrink inwards and not meet the infill as you can see on this piece.

I also find PLA has a tendency to do this on my Mendel but not on HydraRaptor. For a sanity check I built the same object from the same g-code with black ABS.

Notice how much bigger the holes are.

When I was flushing the black out again with the white I noticed that the white had far more die swell and was coming out at about 0.7mm. The black was only about 0.55mm. This means that to extrude at 0.5mm the white is being stretched a lot more, which accounts for why the holes shrink inwards. To test this hypothesis I ran the same g-code again scaling up all the coordinates by 0.6/0.5. This produced a bigger object but the holes are much better.

I then re-sliced the object for 0.6mm filament and that also printed correctly.

So it seems that the white ABS has more die swell than natural or black. In that respect it also reminds me of HDPE. For some reason HydraRaptor is not affected and seems to have less die swell despite having a smaller nozzle, which normally gives more die swell in relative terms because the pressure is higher.

The other thing I discovered is that black ABS does not stick well to PET. It seems a bit greasy.

So with a 0.5mm nozzle if have to build objects at 0.6mm when using white ABS in my Mendel, but with a 0.4mm nozzle on HydraRaptor I can build at 0.375mm or 0.4375mm no problem and holes do not shrink excessively. I am not sure what the difference is, perhaps the length of the nozzle aperture.

No I sort of drill as far as I dare because if I go too far it is ruined and it is not easy to measure. My guess would be a longer final aperture would increase the pressure for a given feed rate so would increase the die swell.

I did experiments a long time ago that showed that smaller apertures give more relative die swell when other parameters are the same. I have never been too sure what the effect of the length is though.

I use a base of gray ABS as a build platform for my machine. I found that black ABS (as supplied by MB) needed a higher temperature to make it bond to it. But when it did bond it would bond so strongly I ended up using chisels, but still ended up destroying base after base.

When it comes to drilling the aperture, I've found the method that leads to the fewest micro-drills breaking(which tends to leave a chunk of the drill bit blocking the hole it has drilled). The way I do it is to drill a hole about a mm deep, and if this isn't deep enough to break through into the 3.5mm hole on the other side. Then use a disc sander to wear away the surface until the hole just drilled is almost gone, then try again. This way the hole depth is kept to a minimum and you don't get constant drill bits snapping off.